Thermally annealed MoS2 nanosheets for highly sensitive NO2 detection: Role of vacancies and heterojunctions.

This work describes the facile, cost-effective fabrication of MoS2-based resistive sensors for low concentration detection of NO2. The sensitive layer consists of films of liquid-phase exfoliated MoS2 nanosheets in aqueous medium deposited by drop-casting onto micromachined silicon substrates with integrated heaters. Thermal treatment of MoS2 conducted during sensor preparation generates high density of S vacancies and induces partial oxidation to non-stoichiometric MoO3-x, resulting in the formation of both S vacancies in MoS2 and O vacancies in the oxide phase. Vacancies act as high-energy binding sites for NO2 and facilitate charge transfer, which enhances the NO2 sensing performance. In addition, the MoS2/MoO3-x heterojunctions formed lead to amplified change in conductivity upon NO2 adsorption, thereby enhancing the overall sensing response. Precise control of oxidation is a key issue, since excessive oxidation may both passivate vacancy sites and decrease the electrical conductivity of the sensing layer through the formation of less conductive oxide phase. The facile fabrication method reported here provided competitive sensor performance: NO2 concentration as low as 0.5 ppm was measured at 200 °C, and 0.25 ppm at 300 °C. Additionally, these sensors showed remarkable selectivity to NO2 with negligible responses to interfering gases (acetone, ethanol and CO). The effect of humidity on sensor response was studied, and it was found that 50% RH humidity significantly enhances the sensing response to NO2. The incorporation of carbon nanotubes to the sensing layers resulted in a faster and higher response to NO2. Detection mechanisms involved are discussed here.